1. Field of the Invention
The present invention relates to an integrating apparatus, and more particularly, to an integrating apparatus suited for use with an on-board audio system, for example.
2. Description of the Related Art
An integrating apparatus is required, for example, by an audio system mounted on an automobile for measuring a noise level with a microphone while the automobile is running, and for reducing an attenuation of audio signals from an audio source, such as a radio receiver, in response to an increase in the noise level so as to raise the level of audio output from a loudspeaker. In the prior art, an output corresponding to the noise level from the microphone is applied to an integrating circuit, and the attenuation of audio signals is varied according to the output from the integrating circuit to determine volume levels. When a small time constant is set for the integrating circuit, the output level of the integrating circuit varies frequently, which results in a drift in the level of audio output from the loudspeaker. In the prior art, therefore, a relatively large time constant is set for the integrating circuit so as to avoid the drift in the audio output level.
The prior art has a disadvantage in that, with the relatively large time constant that is set for the integrating circuit, a long time is taken from the cessation of a noise level signal from the microphone to a lowering of the output level of the integrating circuit. That is, an inconvenience is encountered in which a relatively long time is taken from the lowering of the noise level to the lowering of the audio output level. When the vehicle stops at a toll gate on an expressway, for example, a long time is taken before the lowering of the audio output level. The time taken before the lowering of the audio output level is 5 to 10 seconds, for example.
Another conventional example is disclosed in Japanese Unexamined Patent Publication JP-A 58-38010 (1983). In this prior an example, a microphone detects external noise, and after a rise in the volume of a loudspeaker, the external noise is masked by the speaker sound. This encumbers detection of the actually occurring external noise by the microphone as a result of lowering the volume of the loudspeaker. This prior art example discloses a construction to remedy such a phenomenon. In this prior art example, an integrating circuit is used to avoid the lowering of the volume of the loudspeaker due to the masking of the external noise following a rise in the volume as noted above. This prior art example, however, fails to address the question of reducing the time taken from cessation of a noise level detected by the microphone to the lowering of audio signals from the loudspeaker.